#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstdio>
#include <math.h>
#include <string>
#include <algorithm>
using namespace std;
int n, m, k;
int ans;
int main() {
scanf("%d %d %d", &n, &m, &k);
for (int i = 0;i < k;++i) {
int len = n - (i * 2) * 2;
int wid = m - (i * 2) * 2;
ans += ((len + wid) * 2 - 4);
}
printf("%d\n", ans);
//system("pause");
return 0;
}